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Stalking stones: An overview of canine and feline urolithiasis

Did you know that a new type of urolith has been found in cats? Should you institute preventive therapy if you identify only crystalluria? Is antibiotic therapy automatically warranted in animals with indwelling urinary catheters? This internist revamps your knowledge on diagnosing, treating, and preventing urolithiasis.

Urolithiasis is common in dogs and cats, causing morbidity and, occasionally, mortality. Although renal and ureteral uroliths can sometimes be more difficult to manage, uroliths can usually be successfully treated medically or surgically or by performing lithotripsy. Long-term success is possible. This article will help you diagnose urolithiasis, determine the urolith composition, and develop an appropriate treatment and prevention plan.

CLINICAL SIGNS

Uroliths in small animals occur most frequently in the bladder (urocystolith) but can also be found in the urethra (urethrolith), ureters (ureterolith), and kidneys (nephrolith). Clinical signs of urolithiasis vary according to the urolith's location and any underlying or predisposing conditions. In addition, some uroliths do not produce clinical signs and may be discovered incidentally.

Since the bladder is the most common site for uroliths, the most frequent clinical signs observed are dysuria, hematuria, and pollakiuria with or without inappropriate urination.

If the urethra contains uroliths, the same signs may be observed, with the possible addition of blood dripping from the prepuce or vulva independent of urination. Urethral obstruction may result in unsuccessful micturition attempts, lethargy, anorexia, vomiting, abdominal distention, or pain.

Ureteroliths and nephroliths may be associated with hematuria and abdominal pain as well as lethargy, fever, decreased appetite, and vomiting if an upper urinary tract infection or obstruction is involved.

DIAGNOSIS OVERVIEW

Urolithiasis diagnosis may be aided by historical information, such as a previous occurrence of urolithiasis or an owner's observation of signs compatible with uroliths. A predisposition to urolithiasis may be indicated by patient signalment or pre-existing medical conditions favoring urolith formation. Routine laboratory test (complete blood count, serum chemistry profile, urinalysis) results do not specifically detect uroliths but provide valuable information about predisposing or complicating medical conditions.

Urinalysis

Urinalysis findings, including the pH, evidence of bacterial infection, and the presence of specific crystal types may indicate urolith composition. Urinalysis is best performed within 30 minutes of sample collection. Refrigeration helps preserve the urine sample but can alter the chemical and sediment findings. In particular, time and refrigeration lead to pH and temperature changes, which enhance crystal formation and cause misinterpretation of the urinalysis results.1

Physical examination

You may identify a firm object in the bladder during abdominal palpation or find very small uroliths in voided urine or in penile or vulvar mucoid discharge. Rectal examination findings may reveal firm objects in the pelvic urethra.

Imaging

In most cases, definitively diagnosing urolithiasis requires ultrasonography, survey abdominal or urethral radiography, or contrast radiography. Obtain survey images of the entire length of the urinary tract to identify whether uroliths are present in multiple locations and to identify any factors predisposing the patient to urolith formation, such as infection or neoplasia. Determining the exact number of uroliths with any of the imaging modalities can be challenging in some cases.

Ultrasonography is a good initial imaging tool to detect uroliths; however, it does not reveal the radiodensity or shape of uroliths. In addition, unless a patient's size allows for the use of a rectal transducer, the pelvic urethra cannot be seen with ultrasonography.

Survey radiography can reveal the radiodensity and approximate size and contour of uroliths. It is also generally the most cost-effective and available noninvasive procedure for initially evaluating uroliths.

Although high-frequency (5 to 7.5 MHz) ultrasonography compared favorably with double-contrast cystography in detecting urocystoliths and determining urolith numbers in one study, double-contrast cystography has the added benefit of being a sensitive and specific indicator of urolith shape and size.2,3